1. Field of the Invention
The present invention relates to a feedforward amplifier used to amplify the power of a radio signal in, for example, a base station for a mobile communication system, and also relates to a radio communication apparatus equipped with a feedforward amplifier.
2. Description of the Related Art
Recent mobile communication systems employ a multi-value modulation scheme in light of effective use of a frequency band. A signal obtained by modulation using a multi-value modulation scheme contains information in its amplitude and phase. Accordingly, radio communication apparatuses that employ the multi-value modulation scheme always require a power amplifier having excellent linearity. A feedforward amplifier is known as such a power amplifier. If a feedforward amplifier is used, the signal in an adjacent or nearby channel can be suppressed to a predetermined power level or less.
A feedforward amplifier comprises, for example, an error amplifier and feedforward circuit. The feedforward circuit detects a distortion component contained in a radio signal output from a main amplifier incorporated in the feedforward circuit, and generates, based on the distortion component, a cancel signal that has the same amplitude as the radio signal and a phase opposite to that of the radio signal. This cancel signal is amplified by the error amplifier and then combined with the radio signal, thereby canceling the distortion component contained in the radio signal (see, for example, Jpn. Pat. Appln. KOKAI Publication No. 11-55045).
This feedforward amplifier is generally used when the maximum power is output.
However, in recent cellular communication systems, cell sizes are often varied in accordance with, for example, geographical features or traffic density. In accordance with the cell size, the radio communication apparatus used in a base station is required to vary the transmission power level. Further, the transmission power level of the radio communication apparatus may also be varied in accordance with the distance between the base station and a mobile station, regardless of cell size.
However, if the transmission power level of the radio communication apparatus is low, the level of the distortion component falls within the allowable range of the main amplifier of the feedforward amplifier. In other words, it is not necessary to use the error amplifier. Therefore, in this case, power is wastefully used for operating the unnecessary error amplifier.
In light of the above, it has been proposed to stop the supply of power to the error amplifier when the feedforward amplifier is driven with low power, thereby reducing the power consumption. However, in the feedforward amplifier, the power consumption of the main amplifier occupies the greater part of the whole power consumption. This means that even if the supply of power to the error amplifier is stopped, the total power consumption is not greatly reduced, and the recent demand for saving power is not sufficiently satisfied.